ACTION POTENTIAL & NERVES-TUTORIAL

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Nerves Have Axons, Dendrites and Cell Bodies

• Nerve cells are designed to respond to stimuli and transmit information over long distances
• Nerve cell has 3 parts:
  • Cell body:
    • Has single nucleus
    • Has most of nerve cell metabolism, especially protein synthesis
    • Proteins made in cell body must be delivered to other parts of nerve
  • Axon:
    • Long cylinder, designed to transmit an electrical impulse
    • Can be several meters long in vertebrates (giraffe axons go from head to tip of spine)
    • Has axonal transport system for delivering proteins to ends of cell
    • Transport system has “molecular motors” which ride upon tubulin rails
  • Dendrites:
    • Receive impulses from other nerves
    • In the human brain each nerve is connected to approximately 10,000 other nerves, mostly through dendritic connections

Nerves Transmit Information as Action Potentials

• Action potential is a temporary change in the membrane potential that is transmitted along the axon
  • Usually initiated in the cell body
  • Travels in one direction normally
    • Axon can potentially conduct in both directions, but connections usually prevent this
  • Membrane potential depolarizes (becomes more positive) producing a spike
  • After the peak of the spike the membrane repolarizes (becomes more negative)
    • The potential becomes more negative than the resting potential (negative afterpotential) and then returns to normal
  • The action potentials of most nerves last 5-10 milliseconds (action potentials of cardiac muscle are much longer)
  • The conduction velocity of action potentials is about 1-100 meters/sec (see section on myelin sheath below)

Action Potentials are Initiated by Many Different Types of Stimuli

• Sensory nerves respond to stimuli of many types: chemical, light, electricity, pressure, touch, stretch, etc.
• In the central nervous system (brain & spinal cord) most nerves are stimulated by chemical activity at synapses

Stimuli Must be Above a Threshold Level to Set off an Action Potential

• Very weak stimuli cause a small local electrical disturbance, but do not produce a transmitted action potential
• The graph above shows the electrical disturbances produced by a series of weak (subthreshold) electrical stimuli- the depolarization is very small (less than 1 mV) and no action potential is produced
• When the stimulus strength is increased a little more the threshold is reached and an action potential appears and travels down the nerve as shown in the graph below. Note that the depolarization is much greater, about 110 mV (-70 to +40 mV). The almost flat bottom curve shows the subthreshold response

The Spike of the Action Potential is Caused by Opening of Na Channels

• The Na pump produces gradients of both Na and K ions- both are used to produce the action potential
• Na is high outside the cell and low inside
• Excitable cells have special Na and K channels with gates that open and close in response to the membrane voltage (voltage-gated channels)
• Opening gates of Na channels allows Na to rush into the cell, carrying + charge. This makes the membrane potential positive (depolarization), producing the spike

The Membrane Recovers by Closing the Na Channels and Opening K Channels

• The Na spike does not last long
• Two things bring the voltage back to negative values (repolarization):
  • The Na channels close
    • The have a second slow gate that closes when the voltage becomes positive
  • Potassium channels open when the voltage becomes positive
    • The K gradient is in the opposite direction, so K flows outward making the membrane potential more negative
    • These channels are slower than the Na channels, so Na has the initial advantage, but later K kicks in to bring things back to normal
• Because K permeability is higher than in the resting state the membrane has a negative afterpotential

The Action Potential is Conducted in an All-or-None Manner

• If you take a piece of string and soak it in a salt solution it will conduct electricity
  • If you apply an electrical stimulus at one end you will find that the magnitude of the impulse falls as it travels along the string
  • A simple string would not be very good for long distance conduction of an electrical impulse
• If you apply a stimulus to a nerve the action potential; stays the same magnitude all along the nerve
  • This is called the all-or none-law
  • Nerves are designed for long distance conduction of electrical impulses
• Aspects of the all-or-none law:
  • If the stimulus is too low there is no action potential (this is the “none” part; see threshold below)
  • If the stimulus is above a threshold the action potential is always the same size- it does not get larger for stronger stimuli
  • As the action potential travels along the axon it does not die out, but stays the same size
• As the action potential travels along it triggers the next section of axon to fire
• Like a burning fuse: the heat of the burning section is sufficient to cause the next section of fuse to start burning

Conduction Velocity is Increased by a Myelin Sheath

• Many nerves have an insulating layer called the myelin sheath
  • Produced by Schwann cells in the peripheral nervous system and oligodendrogliocytes in the central nervous system
  • Multiple layers of lipid membranes are wrapped around the nerve
  • Gaps are left every few millimeters: called Nodes of Ranvier
• In a myelinated nerve the impulse jumps from node to node
• Advantage: conduction velocity increases 10 to 100 X
  • Conduction velocity for ordinary nerve = ~1 meter/sec (depends upon diameter)
  • Conduction velocity for myelinated nerve = ~100 meters/sec
• Demyelinating diseases cause sever nerve defects
  • Autoimmune diseases: immune system attacks nerves
  • Multiple sclerosis: demyelination in the central nervous system -> delayed or blocked conduction in some nerves

The Nerve Has a Refractory Period

• After a nerve has fired there is a refractory period during which it cannot be stimulated- it must recover before it can fire again
• To recover the second, inhibitory gate in the Na channel must open: this resets the channel
• In the graph above the nerve is stimulated at 0.25 milliseconds and this produces the first action potential
• It is stimulated a second time at 3.5, 4, 4.5 and 5 milliseconds, but nothing happens- the nerve is refractory
• A second stimulus at 5.5 milliseconds produces a second action potential- the nerve has recovered by this time
• The refractory period controls the rate at which a membrane can fire (long refractory period -> slow firing rate)

Some Drugs and Toxins Inhibit Na Channels

• Local anesthetics such as lidocaine block Na channels and inhibit action potentials
  • This blocks the conduction of pain fibers
• The puffer fish toxin, tetrodotoxin, is a powerful inhibitor of Na channels
  • Inhibits action potentials and blocks nerve conductance
• Blocking conductance of certain nerves (i.e., those that control respiration) can be lethal

Muscle Membranes Also Conduct Action Potentials

• Skeletal, cardiac and smooth muscle membranes also conduct action potentials
• They operate in the same way as nerve membranes but some of the details are different- for example, in cardiac muscle Ca ions are very important and the action potential duration is much longer

More Information:

Eric Chudler of the University of Washington has created an exciting Neuroscience for Kids website. Check out his action potential page.

The Madonna computer program was developed at UC Berkeley by Robert Macey and George Oster. If you are interested you can download a trial copy. The program is easy to learn, but it helps if you have had a little calculus. The Hodgkin & Huxley nerve axon equations are described at the website for Macey & Oster’s computer simulation class.